The normal sequence of events at a loading bay is that a vehicle (a semi-trailer, a drawbar trailer, or a lorry) is driven up to a loading bay and the driver contacts staff in the building. The staff then open the loading bay for the driver.
The loading bay has a door, usually of the ‘up and over’ type, and a dock leveler. The dock leveler is a powered ramp which is adjusted in height to accommodate differences in vehicle loading heights and to enable forklifts to drive from the building into the vehicle.
There is a risk that if the trailer is allowed to pull away or move prematurely from these loading bays then the forklift could fall from the dock leveler. To prevent such accidents, or simply to prevent such premature vehicle departures, several safeguards are currently employed. These systems include:
a) Traffic lights, some of which are linked to the dock leveler. This does not prevent the driver from moving off but relies upon him obeying the lights. These systems are quite expensive.
b) A traffic barrier system which retains the vehicle at the loading bay until a barrier is raised. This also relies upon correct procedure and is expensive.
c) An inbuilt wheel blocking device which is installed in the ground at each bay and which is raised to prevent the vehicle from leaving until loading is complete. This is very expensive and also relies upon correct procedure to be effective.
d) A wheel clamp which is fitted when the vehicle arrives and which provides a physical and visual barrier to indicate to the driver and he/she should not drive off. This also relies upon procedures and is cumbersome to operate.
e) A trapped key safety interlock which actively prevents the vehicle from being driven away from a loading bay during loading/unloading, the apparatus comprising a safety device which is complementary to a coupling on the vehicle and which when fitted to the coupling prevents the vehicle from being driven away, e.g. by causing the air supply for the trailer's air brakes to be maintained in a fail (disconnected) condition—a condition in which the brakes are automatically engaged. Such a safety device is disclosed in, for example, EP1493620, the whole contents of which are incorporated herein by way of reference.
The trapped key safety interlock system includes a locking device which is operable to prevent removal of the safety device from the coupling and which prevents removal of a key from the locking device until the locking device is so operated. Only then can the released key be used to operate a device, such as a switch or a door lock, to enable the use of a loading bay.
This safety device system then further prevents removal of the key from the switch or door lock until that is operated to disable use of the loading bay (since a corresponding key interlock mechanism is provided at that location too). Therefore, only once the loading bay is disabled can that key be removed therefrom for then again unlocking the earlier mechanism that is causing the trailer's braking system to be engaged.
This trapped key safety interlock system has now been widely adopted, but a potential breach of its interlock can occur if a user manually interferes with the internal locking mechanism of the device before locking it onto the trailer's coupling, thus freeing the key either without first engaging the vehicle's braking system or without first disabling the loading bay.
The present invention therefore looks to provide a tamper resistant version of trapped key safety interlock.